The present invention relates to the production of thin films of complex silver iodo solid electrolytes on a substrate, and is particularly concerned with the deposition on a substrate such as a ceramic substrate or a metallic substrate, e.g. forming an electrode of a coulometer or a battery, of a thin stable film of oxy-anion-substituted silver iodide solid electrolyte, such as Ag.sub.19 I.sub.15 P.sub.2 O.sub.7, such film having good ionic conductivity at ambient temperature and particularly containing iodine in stoichiometric proportion, substantially without any excess or deficiency thereof, and to procedure for producing such films, and for incorporating such films into devices such as batteries, coulometers, memory devices, and the like.
The solid electrolyte devices have several distinct advantages over those based on liquid electrolytes. These include (1) the capability of pressure-packaging or hard encapsulation to yield extremely rugged assemblies, (2) the extension of the operating temperature range since the freezing and/or boiling-off of the liquid phase, which drastically affect the device performance when employing liquid electrolytes are no longer a consideration, (3) solid electrolyte devices are truly leak-proof, (4) they have long shelf life due to the prevention of the corrosion of electrodes and of loss of solvent by drying out which occur when using liquid electrolytes, and (5) solid electrolytes permit micro-miniaturization.
All of the above considerations have led to a growing use of solid electrolytes. Solid state batteries and timers are already available which employ the solid electrolyte as a cylindrical pellet with suitable electrodes on either side. However, this kind of geometry leads to somewhat poor solid-solid contacts and these devices tend to have high internal resistances and polarization losses. These problems have been overcome by the use of thin films as the electrolytes, since thin films deposited on top of each other have excellent contacts and should also be able to withstand shocks, acceleration forces and spin rates without undue damage. Furthermore, the power requirements of electronic devices are being continually reduced by the development of more sophisticated micro-electronic circuit components. If such advances are to be utilized to their fullest extent, there must be a corresponding reduction in the size of the power sources which can only be accomplished by employing thin film batteries.
Several solid electrolytes are known to exhibit good ionic conductivity, some of which exist in the form of thin films. Oxide ion conductors such as zirconia are operated at high temperatures due to their low conductivity at ambient temperatures. Chloride ion conductors such as PbCl.sub.2 and BaCl.sub.2 have similar temperature restrictions. Most silver halides such as AgBr, AgCl, and AgI also show low room temperature ionic conductivity but new compounds, based on the modifications of silver iodide (AgI), have recently been synthesized which show high ionic conductivity at room temperature. Such modifications of silver iodide involve partial replacement of either the cation (silver) or the anion (iodine) or both by some other suitable ions.
Cationic substitution leads to compounds of the type MAg.sub.4 I.sub.5 where M may be either Rb or K or NH.sub.4. The most stable of these compounds, which also shows the highest room temperature ionic conductivity, is RbAg.sub.4 I.sub.5 but its use creates its own problems. It is known that the thin films of RbAg.sub.4 I.sub.5 generally contain excess iodine which causes corrosion of the electrodes which in turn reduces the shelf life of the devices using these films.
On the other hand, a partial anionic substitution in silver iodide gives rise to compounds which are known as oxy-anion-substituted silver iodide electrolytes, a typical example of which is Ag.sub.19 I.sub.15 P.sub.2 O.sub.7. These latter solid electrolytes are all known to be stable at ambient temperatures and humidity for long periods of time and show good ionic conductivity. However, employing conventional techniques of vacuum evaporation, flash evaporation and sputtering, for depositing thin films of these electrolytes on a substrate have heretofore been largely unsuccessful since they have yielded films of these compounds of relatively low ionic conductivity, with the typical value being between 10.sup.-5 to 10.sup.-6 (ohm - cm).sup.-1. The main factor in the production of such undesirable low ionic conductivity films has been the failure of the above noted prior art techniques for depositing the vapors of such compounds on the substrate in stoichiometric proportions.
Thus, in the successful fabrication of stoichiometric films of the above noted oxy-anion-substituted silver iodide electrolytes, the major consideration is that the net condensation of the vapors onto the substrates be stoichiometric. The net condensation involves the total amount of vapors arriving at the substrates, what fraction will deposit on them and what fraction will re-evaporate from the substrate.
It is generally believed that the conventional methods of film formation fail with Ag.sub.19 I.sub.15 P.sub.2 O.sub.7 and other oxy-anion-substituted silver iodide compounds due to the great differences that exist in vapor pressures of the silver-containing, iodine-containing and oxy-anion containing constituents when these electrolytes are heated. It must be remembered that these compounds have incongruent melting points. Therefore, they decompose on evaporation and do not deposit on the substrates in the proper stoichiometric proportions.
It is accordingly an object of the invention to produce thin stable films of complex silver iodo solid electrolytes on a substrate, such films having good ionic conductivity at room temperature and maintaining such conductivity over extended periods of use and storing. A particular object of the invention is the provision of novel procedure for depositing such films, particularly oxy-anion-substituted silver iodide solid electrolyte films, such as Ag.sub.19 I.sub.15 P.sub.2 O.sub.7, on a substrate, by flash evaporation or RF sputtering, under conditions such that the condensation of the vapors unto the substrate is substantially stoichiometric, particularly with respect to the iodine content of the electrolyte, and avoiding an excess or deficiency of iodine in the deposited solid electrolyte film, thereby producing electrolyte films which are stable and have good ionic conductivity at room temperature. A still further object is the production of improved thin solid electrolyte films of the aforementioned type, deposited on a substrate, and which are particularly useful in the formation of thin-film batteries, coulometers, memory devices, and the like.